The invention relates to an optical transmission network and a method for transmitting binary information in successive time slots of a time division multiplex frame format in the optical transmission network which has a number of terminals and a central unit. At least one optical fibre is connected to the central unit and spreads to the terminals in a tree-like structure via at least one beam splitter and further optical fibres. An address is allocated to each terminal. Synchronization of the terminals is accomplished by phase-locking the terminal to the central unit in dependence upon the signal delay time between the terminal and the central unit. The transmitting level of the terminal is adjusted in dependence upon the line attenuation between the terminal and the central unit. The frame format has a number of time slots for line-switched information as well as a number of bits for control and synchronization purposes.
Optical transmission networks have been described in EP-A-O 318 331. It is well-known to use beam splitters in such optical transmission networks in order to attain a tree-like structure and thus to ensure that each subscriber connected to the network can start communication via the central unit or can be integrated into communication by the central unit. Such a tree-like structure clearly defines the transmission time being required for the path between the subscriber and the central unit, which is a fundamental requirement for the synchronization of transmission. Beam splitters or optical splitters represent an attenuator in both directions of transmission within such a network, which makes the level arriving via the transmission path dependent on the length of the path and the number of beam splitters.
In such optical transmission networks, time multiplex transmission in time slots is standard, whereby each terminal usually is organized to have a time slot of its own within which communication between the terminal and the central unit can occur. Moreover, transmission usually is organized to provide--apart from the respective time slots--a number of bits for packet-oriented communication as well as a number of bits for internal control and synchronization purposes. The packet section of the frame is usually triggered by a main unit- or central unit- command. Thus, the optical fibre network is designed as a passive network, and within the transmission path, apart from optical fibres and passive beam splitters, no further equipment is provided to modify the transmission path.
In order to assign to each terminal an adequate time window within the time multiplex frame format, usually a switching-on and configuration procedure has to be preformed to be able to integrate all terminals being activated into the transmission network. The optical transmission network already known from EP-A-O 318 331 has such a switching-on and configuration procedure triggered centrally; and, depending on the distance to the terminal, for each terminal to be newly installed one or two frames are used for signal delay time measuring purposes. For the adjustments of the terminal, each frame has a first section as a "housekeeping" channel. Subsequently, each active terminal has to have an address which enables it to be addressed by the central unit directly and unconfusingly, and a time slot will have to be assigned thereto on which it is able to send and receive its line-switched information in each frame. Making the address and time slot number identical is useful.
An active terminal, which is a terminal integrated into the network, will have to know the signal delay time between the terminal and the central unit. The delay time resulting from the signal delay is the time which has to pass between the start of the received frame coming from the central unit and the start of the terminal's own transmitting to enable its own transmission signal to insert itself into the appropriate time slot of the frame.
Naturally, signals can only be received if the line attenuation allows signal detectability. Therefore, it will be necessary to adapt the transmitting level of a terminal to the given line attenuation in a manner which enables unmistakable signal identification and processing in comprehensive optical networks. In order to assure the reliability of signal intelligibility throughout the whole network, it will be advantageous if the transmitting level is adjusted so that the level received by the central unit is equal for all terminals.
In a well-known embodiment of the optical transmission network, the switching-on and configuration procedure is initiated by the central unit. This means that the central unit has to check--throughout the whole network--the possibility of connecting a further terminal in regular time intervals. The central checking procedure and the information about the values to be adjusted by the terminals must be performed via the "housekeeping" channel much more often than new terminals are to be connected. If such a partial blockage of the central unit for unnecessary sampling of information via the "housekeeping" channel is to be avoided, it would be required that new terminals only be connected at certain times. Thus, the well-known configuration could not do without the "housekeeping" channel.
In larger networks in particular, it will not be easily possible to connect extensions at random instants if manual intervention in the transmission processing is to be avoided. In addition, a terminal, when being switched on due to a switching-on and configuration procedure from the central unit, will have to be able to transmit a given address, which is a hardware feature of the respective terminal. Therefore, this given address will have to be made known to the central unit because, if following the conventional procedure, a switching-on and configuration procedure will not be successful without the address. Under no circumstances may this given address be assigned several times by the producers of terminals, which results in producers, high expenditure for administration and assignment of addresses, and, usually, in relatively long addresses for safety reasons. Thus, given addresses have to be made available for the central unit to initiate a switching-on and configuration procedure after such manual handling.